1. Field of the Invention
The invention relates to the fabrication of fiber-reinforced phthalonitrile composites and, in particular, to fiber-reinforced phthalonitrile composites made by a method wherein the curing agent is selected to have low reactivity with the phthalonitrile monomer.
2. Description of the Related Art
Phthalonitrile resins made using amine curing agents are described in Keller, T. M. and Price, T. R., "Amine-Cured Bisphenol-Linked Phthalonitrile Resins", J. Macromol. Sci.-Chem., A18(6), pp. 931-937 (1982), U.S. Pat. No. 4,408,035 to Keller, U.S. Pat. No. 5,003,039 to Keller, U.S. Pat. No. 5,003,078 to Keller, U.S. Pat. No. 5,004,801 to Keller, U.S. Pat. No. 5,132,396 to Keller, U.S. Pat. No. 5,139,054 to Keller, U.S. Pat. No. 5,208,318 to Keller, U.S. Pat. No. 5,237,045 to Burchill et al, U.S. Pat. No. 5,292,854 to Keller and U.S. Pat. No. 5,350,828 to Keller et al., the disclosures of which are incorporated herein by reference.
Fiber-reinforced composites are typically made by heating a phthalonitrile monomer to its melt stage, adding a curing agent to the melted monomer to form a prepolymer mixture and then impregnating or coating a fibrous material such as carbon fiber with the melted prepolymer mixture. The fiber-containing prepolymer mixture is then allowed to cure at an elevated temperature to form the fiber-reinforced composite.
In creating fiber-reinforced composites by the method described above, it is necessary that the phthalonitrile prepolymer melt and flow easily to completely adhere to and impregnate or coat the fibrous material. A problem that often arises is that the high temperature necessary for melting the phthalonitrile monomer also speeds the curing reaction, particularly if a fast-reacting amine such as 1,3-bis(3-aminophenoxy)benzene is used as the curing agent. If the curing reaction proceeds too rapidly, the increase in the viscosity of the prepolymer associated with the curing prevents the prepolymer from flowing freely and completely permeating and impregnating or coating the fibrous material, resulting in a defective or resin-poor composite. This problem may be overcome by using less of the amine curing agent (as shown, for example, in Sastri et al, "Phthalonitrile-Carbon Fiber Composites" Polymer Composites, December 1996, Vol. 17, No.6, pp 816-822 and Sastri et al "Phthalonitrile-Glass Fabric Composites", Polymer Composites, February 1997, Vol. 18, No. 1, pp 48-54, the disclosures of which are incorporated herein by reference). However, using too little of the curing agent results in insufficient and incomplete curing of the phthalonitrile resin. With a fast-reacting amine curing agent such as 1,3-bis(3-aminophenoxy)benzene, the processing window between too much curing agent and not enough curing agent may be narrow. A narrow processing window can increase processing costs because greater care must be taken to insure that the right amount of curing agent is used.
Another problem that arises in the fabrication of fiber-reinforced phthalonitrile composites is that the high temperatures used in many modern methods of composite fabrication tend to be above the decomposition or volatilization temperature of many of the amine curing agents that have been traditionally used in the formation of phthalonitrile polymers. Thus, there is a great need for amine curing agents that are thermally and oxidatively stable at very high temperatures (up to about 375.degree. C.).
An alternative method for creating a fiber-reinforced composite by penetrating and impregnating or coating a fibrous material with a phthalonitrile prepolymer is to dissolve the phthalonitrile prepolymer in a solvent and then impregnate or coat a fibrous material with the solvent/prepolymer mixture. However, the solvent method has its own set of problems such as the potential problem of creating undesirable voids in the composite by the action of entrapped volatilized solvent molecules. Moreover, the solvent method involves additional elaborate processing steps and problems relating to removal and disposal of the solvent.
In methods of creating fiber-reinforced composites by prepreg consolidation and filament winding, a long fibrous tow or filament is run through or dipped into a container of a prepolymer melt or prepolymer/solvent mixture so that the tow or filament becomes thoroughly coated with the prepolymer, forming, for example, a prepreg tape or a preform. In these methods of creating fiber-reinforced composites, it is crucial that the prepolymer not cure too quickly before the coating process is complete.